The formation of bacterial biofilms is increasingly recognised as the leading cause of chronic infections. It limits the application of implant materials including catheters, heart valves, or orthopaedic prostheses. It is generally assumed that the infection persists because bacteria organised as biofilms escape the host defence mechanisms. Nevertheless, when studying patients with infected implants, we found a massive infiltration of leukocytes particularly polymorphonuclear neutrophils, PMN, into the site of infection, which led to the question, whether the PMN interact with the bacterial biofilm or not.

The interaction of human PMN with Staphylococcus aureus biofilms was studied in vitro.

S.aureus was cultivated on glass cover slips for various times under conditions allowing formation of biofilms. Adherence of PMN to biofilms and phagocytosis of the bacteria were observed by confocal laser scan microscopy and time lapse video microscopy.

Migration of PMN on and into the biofilm was identified as being phagocytosis, apparent as uptake of bacteria into the cell. Concominantly, in the wake of migrating PMN bacteria depleted zones appeared, which increased in size with time. In addition to phagocytosis, release from PMN of DNA and also of elastase was seen, suggesting the formation of neutrophil extracellular traps (NETs). So far, the signal for DNA release and NET formation has not been identified; of note is, however, that they occurred preferentially on established “old” biofilms and in the absence of the opsonising human serum, while phagocytosis was most efficient with developing “young” biofilms.

Taken together, our data provide evidence that bacteria in biofilms are not entirely protected against host defence but that phagocytosis is still possible, especially when the biofilm is opsonised with human serum. Whether NET formation also contributes to bacteria killing in biofilms cannot be decided as yet but remains an attractive alternative.